Polyamide substrates, such as nylon carpeting and upholstery fabric and similar wool substrates, are subject to staining by a variety of agents. Acid dyes are especially troublesome staining agents, e.g. FD&C Red Dye No. 40, commonly found in soft drink preparations. Prior proposals for inhibiting staining of polyamide substrates by acid dyes include application of sulfonated phenolformaldehyde condensates, alone, or in combination with hydrolyzed maleic anhydride polymers or polymers of methacrylic acid, acrylic acid, or itaconic acid, or combinations of the same. It has been reported to be essential that the acid dye stain-resist agent contain acid groups in order to impart water-solubility to said condensates and polymers; i.e. sulfonic acid groups in the sulfonated phenol/formaldehyde condensates and carboxylic acid groups in the polymers of maleic anhydride, methacrylic acid and the like. It has also been reported that as the ratio of units containing one --SO3X radical to units containing no --SO3X radicals increases, the product becomes a better stain blocker. In addition, it has been reported that in order to be effective, stain-resists must be applied below pH of 4.5, preferably below 3.0; however, operating at such low pH has the potential for causing corrosion of equipment as well as safety and environmental problems.
Unfortunately, polyamide substrates treated with sulfonated phenolformaldehyde condensates suffer from discoloration when exposed to ultraviolet light (UV), limiting their usefulness. The UV-induced discoloration has been found to be primarily a result of discoloration of the sulfonated condensate itself, since polyamide substrates not treated with the condensates undergo significantly less discoloration upon similar exposure to UV light. It has been reported that when sulfonated phenol-formaldehyde condensates are applied to substrates at pH's between 2.5 and 7.0, greater discoloration caused by exposure to UV light was observed for the substrates to which the condensates had been applied at higher pH. It has also been reported that polyamide carpet treated with sulfonated phenolformaldehyde condensates had improved UV lightfastness when post-treated with acid solution but significantly worse UV lightfastness when post-treated with base solution. The stain-resistance of the treated carpet was not diminished with either treatment.
A formaldehyde/naphthol condensate having no carboxylic or sulfonic acid groups and having restraining effects to anionic dyes has been reported. However, our testing of the resulting compound, 2,2'-dihydroxy-1,1'-dinaphthylmethane, by the procedures described in the present specification showed it to be unacceptable as a stain-resist agent for polyamide fibers. On the other hand, base-catalyzed condensation products of a bis(hydroxy-phenyl)sulfone and formaldehyde have been described in the prior art as melt additives useful in improving the flame retardancy and thermal stability of polyamide fibers.